一个使用Systemd的例子

init系统.

linux操作系统首先从BIOS启动开始，接下来进入bootloader（桌面系统现在一般用Grub），bootloader载入内核并进行初始化。内核初始化的最好一步就是启动PID为1的init进程。init进程是以守护进程的方式存在，是所有其他进程的祖先。

init系统的演进路线经历了sysvinit->upstart->systemd。

systemd系统简介

借用一下帮助文档里的关系图(man bootup)

       local-fs-pre.target
                |
                v
       (various mounts and   (various swap   (various cryptsetup
        fsck services...)     devices...)        devices...)       (various low-level   (various low-level
                |                  |                  |             services: udevd,     API VFS mounts:
                v                  v                  v             tmpfiles, random     mqueue, configfs,
         local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)      debugfs, ...)
                |                  |                  |                    |                    |
                \__________________|_________________ | ___________________|____________________/
                                                     \|/
                                                      v
                                               sysinit.target
                                                      |
                 ____________________________________/|\________________________________________
                /                  |                  |                    |                    \
                |                  |                  |                    |                    |
                v                  v                  |                    v                    v
            (various           (various               |                (various          rescue.service
           timers...)          paths...)              |               sockets...)               |
                |                  |                  |                    |                    v
                v                  v                  |                    v              rescue.target
          timers.target      paths.target             |             sockets.target
                |                  |                  |                    |
                \__________________|_________________ | ___________________/
                                                     \|/
                                                      v
                                                basic.target
                                                      |
                 ____________________________________/|                                 emergency.service
                /                  |                  |                                         |
                |                  |                  |                                         v
                v                  v                  v                                 emergency.target
            display-        (various system    (various system
        manager.service         services           services)
                |             required for            |
                |            graphical UIs)           v
                |                  |           multi-user.target
                |                  |                  |
                \_________________ | _________________/
                                  \|/
                                   v
                         graphical.target

systemd 使用

• 显示某个服务的实时日志

journalctl --no-pager -f -u hello.service   

• 编写服务配置文件

~ cat /root/hello
NAME=xxxxyyyyzzz

• 编写服务脚本

~ cat /usr//local/bin/hello.sh
#!/bin/bash

name=$1
while true
do
    echo Hello World $name
    echo "*" >> /root/hello.log
    sleep 1
done

• 编写服务的systemd配置文件 可以放到/lib/systemd/system目录，也可以放到/etc/systemd/system目录下，/etc/systemd/system的优先级更高

[Unit]
Description=Hello world Service

[Service]
EnvironmentFile=/root/hello
ExecStartPre=-/usr/bin/touch /root/hello.start
ExecStartPre=-/usr/bin/pkill hello.sh
ExecStart=/usr/local/bin/hello.sh ${NAME}
#Type=forking
Restart=on-success
RestartSec=5

• 设置服务的cgroups参数

systemctl set-property hello CPUShares=1600 MemoryLimit=600M --runtime

• systemd-run examples

systemd-run --user --on-calendar '2017-08-12 14:46' /bin/bash -c 'echo done >/tmp/done'

service/socket example

• with golang Httpserver
• with python Httpserver